Laser Excitation Spectroscopy

1
Laser Excitation Spectroscopy of SrSH and
CaSH By Michael Dick, P. M. Sheridan, Peter
Bernath, and J.- G. Wang
2
Motivation Ca and Sr Polyatomics

• Oxides, halides and hydroxides containing Ca and
  Sr have been well studied.
• Ca and Sr polyatomics are not as well
  characterized but are interesting
• Light, few isotopes, main isotopes have no
  nuclear spin.
• Metals ablate easily and form molecules readily.
• Follow the progression of electronic states as
  the symmetry of the ligand decreases.

SrNH2, SrSH Asymmetric Top
SrF Diatomic
SrCCH Linear
SrCH3 Symmetric Top

• While the ground states for many of these
  polyatomic molecules have been examined, little
  or no information exists about the excited states.

3
CaSH/SrSH A-X

• Electronic Transition
• s ? p? (in plane) promotion on the metal atom -
  perpendicular transition
• Appearance like a Hunds case (a) 2 ? - Hunds
  case (b) 2S transition.
• Previous Work
• CaSH
• Low resolution by Bernath and coworkers.
• High resolution by Bernath and coworkers
• SrSH
• Low resolution by Bernath and coworkers.

4
CaSH/SrSH B-X

• Electronic Transition
• s ? p? (out of plane) promotion on the metal atom
  - perpendicular transition
• Appearance like a Hunds case (a) 2? - Hunds
  case (b) 2S transition.
• Previous Work
• CaSH
• Low resolution by Bernath and coworkers.
• High resolution by Steimle and coworkers,
  including Stark spectroscopy.
• SrSH
• Low resolution by Bernath and coworkers.

5
CaSH/SrSH C-X

• Electronic Transition
• s ? ps promotion on the metal atom - parallel
  transition.
• Appearance like a Hunds case (b) 2S - Hunds
  case (b) 2S transition.
• Previous Work
• CaSH
• Low resolution by Bernath and coworkers.
• SrSH
• Low resolution by Bernath and coworkers.
• No high resolution work!

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The Laser Ablation Source
Single Mode Ring Dye Laser
I2 Cell w/ PMT
Preamp
Rod Rotator
Scope
Preamp
Boxcar
PMT
Delay Box
Pulsed Valve Power Supply
Pump
YAG 3rd Harmonic
PC
Trot 48 K

• Gas mix 7.5 H2S in argon at a backing pressure
  of 100 psi.

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CaSH C-X

• Expected Hunds case (b) 2S - Hunds case (b) 2S
  structure.

8
SrSH C-X

• Expected Hunds case (b) 2S - Hunds case (b) 2S
  structure.

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C-X Energy Level Diagram


Near Prolate Asymmetric Top a type
transitions ?Ka0, ?Kc1 Diagram shows only Ka
0 subband. Other Ka?0 subbands possible, but
weaker due to cooling. Ka0 subband will appear
like a Hunds case (b) 2S - Hunds case (b) 2S
transition. Each rotational level is split into
F1 and F2 components by the spin-rotation
interaction.
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CaSH/SrSH Ka0 Subband
CaSH
SrSH

• SrSH - P1/R2 branches smaller spacing, P2/R1
  branches larger spacing
• CaSH - P1/R2 branches larger spacing, P2/R1
  branches smaller spacing
• Indicates sign change in spin rotation
  interaction.

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CaSH/SrSH Ka1 Subband
CaSH
SrSH

• Red arrows indicate unassigned lines likely
  arising from Ka 1 subband.
• Extra lines more apparent in CaSH due to
  increased signal to noise ratio.

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CaSH/SrSH Molecular Constants

C state molecular constants for CaSH and SrSH

• Ka0 subband and pure rotational data fit to
  Watsons S-Reduced asymmetric top Hamiltonian
  (Picketts Program).
• The pure precession relationships indicate that
  (ebbecc)/2 should be negative.
• CaSH appears to be following this relationship,
  while SrSH does not. The C state of SrSH is
  perturbed.

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B-X Energy level diagram


c type transitions ?Ka1, ?Kc0,2 Diagram
shows a Ka(1-0) subband Ka(1-0) subband will
appear like a Hunds case (a) 2? - Hunds case
(b) 2S transition. The F1 and F2 components in
the B state will exhibit a large splitting due to
eaa ( -19.7 cm-1) Each Ka1 rotational level is
further split by asymmetry doubling.

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SrSH B-X

• Each spin component has the expected Hunds case
  (a) 2? - Hunds
• case (b) 2S appearance with 1B and 3B spaced
  branches.
• The approximate value of eaa( -15 cm-1) is close
  to that predicted by the pure precession
  relation.

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SrSH Assigned Spectrum

• Lower state combination differences confirm the
  above assignments.
• The F2 component has not yet been assigned.

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A-X Energy level diagram


b type transitions ?Ka1, ?Kc1 Diagram
shows a Ka(1-0) subband Ka(1-0) sub band will
appear like a Hunds case (a) 2? - Hunds case
(b) 2S transition. The F1 and F2 components in
the A state will exhibit a large splitting due to
eaa ( 19.7 cm-1) Each Ka1 rotational level is
further split by asymmetry doubling.

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SrSH A-X

• The approximate value of eaa(5 cm-1) is not
  close to that predicted by the pure precession
  relation(19.7 cm-1).

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Perturbation Summary

• The eaa value of the A state of SrSH is reduced
  from 19.7 cm-1 (pure precession) to 5cm-1. A
  similar reduction was also observed in the A
  state of CaSH.
• Reduction in CaSH attributed to orbital mixing of
  the A and C states.
• This may contribute to the change in sign of
  (ebbecc)/2 in the C state of SrSH.
• The B state of SrSH appears to be less perturbed
  as the calculated
• eaa(-19.7 cm-1) is close to the observed value
  (-15 cm-1)
• More work is needed to fully understand these
  various perturbations.